Zooplankton of the St. Lucia Estuary during the current drought cycle: a
INTRODUCTION
An inverted salinity gradient has persisted, with the occurrence of hypersaline conditions in the northern areas of the lake. This knowledge would contribute significantly to the synthesis of information on the secondary producers of St.
MATERIALS AND METHODS
- Physico-chemical variables
- Pelagic and benthic microalgae
- Zooplankton
- Statistical analyses
Where possible, measurements were taken at both the surface and the bottom of the water column. Groundwater samples were collected at each of the five stations during each sampling season.
RESULTS
- Physical environment
- Phytoplankton and microphytobenthic biomass
- Zooplankton
- Community structure
During the open and reclosed mouth phases, biomass was generally uniform across all stations (Table 1.1). Abundance of the mysid Mesopodopsis africana was highest at the mouth and Esengeni during the closed-mouth phase, but was practically zero throughout the system during the open-mouth phase.
DISCUSSION
Spatial and temporal variations in the diet of the mysid Mesopodopsis africana in the St. Carrasco NK, Perissinotto R (2010b) Spatial and temporal variations in the diet of the mysid Mesopodopsis africana in the St. Carrasco NK, Perissinotto R (2011a) Temperature and salinity tolerance of the dominat mysid in the freshwater deprived St.
Carrasco NK, Perissinotto R (In review) Diet of the dominant fish species in the freshwater deprived St.
Temperature and salinity tolerance of Mesopodopsis africana in the freshwater-
INTRODUCTION
The Lucia Estuary, the largest estuarine lake in Africa (Fielding et al. 1991, Cyrus and Vivier 2006), is part of the iSimangaliso Wetlands Park (formerly Greater St. Lucia). This change in the system's watershed has exacerbated the severity of the current drought cycle. The reversed salinity gradient has been nearly continuous since 2002, with values ranging from 1.8 in the Narrows to > 300 in the upper estuary.
Grindley previously recorded the salinity of many of the common zooplankton taxa living in the estuary, and suggested that M.
MATERIALS AND METHODS
- Study area
- In situ sampling
- Physico-chemical variables
- Mysid sampling
- Experimental study
- Collection and maintenance of mysids
- Salinity and temperature tolerance experiments
- Statistical analysis
Mysids for experiments were collected using the same epibenthic sled mentioned above, which was dragged across the mouth of the St. Since the purpose of the acclimation experiments was to determine whether the tolerance limits of the mysids seen in the shock experiments could be stretched, the following range of salinities was used in the acclimation experiments and 65. The plastic containers of 500 ml were covered with a 200 µm mesh and immersed in each of the aerated buckets, which were then immersed in the water baths at different temperatures.
In addition, Dunnett's T3 post hoc test was used for comparisons between treatments, as this text assumes a non-uniform distribution of residuals.
RESULTS
- Physico-chemical environment
- Mysid abundance
- Temperature and salinity shock and acclimation experiments
In shock experiments, temperature and salinity and the interaction between these variables significantly affected mortality (RM-ANOVA, F18 = 60.7, p < 0.001). For temperature, mortality in the 20 and 30°C treatments was significantly different from that in the 10 and 40°C treatments (post hoc Dunnets T3, p < 0.001). Both temperature and salinity and the interaction between these variables had a significant effect on the mortality of mysids (RM-ANOVA, F15 = 18.2, p < 0.001).
As in the shock experiment, mortality in the temperature treatments at 20 and 30ºC was significantly lower than that in the temperature treatments at 10 and 40ºC (post hoc Dunnets T3, p < 0.001).
DISCUSSION
Temporal changes in the δ13C signatures of primary producers in Charters Creek were significantly different (ANOVA, F p < 0.05). In the wet season (Figure 5.2a), Pseudodiaptomus stuhlmanni and Acartia natalensis contributed up to 52 and 46% of the diet, respectively. Carrasco NK, Perissinotto R, Miranda NAF (2007) Effects of silt loading on feeding and mortality of the mysid Mesopodopsis africana in St.
Gasparini S, Castel J (1997) Autotrophic and heterotrophic nanoplankton in the diet of the estuarine copepods Eurytemora affinis and Acartia bifilosa.
In situ feeding rates and grazing impact of Mesopodopsis africana in the St
INTRODUCTION
The diel-vertical migration characteristic present in some species is widely accepted as a mechanism by which mysids reduce the risk of predation (Gliwicz 1986, Boscarino et al. 2009). Drought conditions were exacerbated by the channelization of the Mfolozi River and the subsequent diversion of its fresh water away from St. Lucia, like most estuarine zooplankton communities in southern Africa (Wooldridge 1999), is dominated by typical estuarine taxa such as Pseudodiaptomus stuhlmanni, Acartia natalensis and Mesopodopsis africana, which together often contribute more than 90% of the total zooplankton abundance (Carrasco et al. 2010: Chapter 1).
The overarching aim of the study was to investigate the trophic role of this key species within the St.
MATERIALS AND METHODS
- Study site and mysid collections
- Pelagic and benthic microalgae
- Zooplankton grazing experiments
- Grazing impact and daily ration
Pigment content in acetone solution was measured with a Turner Designs 10-AU fluorometer and expressed as hl a equivalents per individual (ng pigm.ind-1). Gut evacuation rate was derived from the slope of the regression of gut pigment change versus time (Perissinotto and Pakhomov 1996). Assessment of the degree of intestinal pigment destruction was determined using a two-pronged approach (Perissinotto 1992).
Grazing impact was expressed as a percentage of the microalgae standing stock consumed per day as well as impact on the primary production.
RESULTS
- Physico-chemical environment and microalgae
- Zooplankton abundance and biomass
- Mysid grazing experiments
In situ grazing rates at Munden were therefore greater than those recorded at Charters Creek. Grazing rates for Mesopodopsis africana at Charters Creek were approximately 16.4 ng pigm.ind-1.h-1, while at the mouth they were 23.2 ng pigm.ind-1.h-1. Considering the integrated values of gut pigment content and abundance at each time interval throughout the 24-h sampling period, the mysid grazing effect on total microalgal biomass was estimated to be 0.146 mg pigm.m–3.d–1 at Charters Creek and 1.58 mg pigm .m–3.d–1 at the mouth.
Using the intake rates obtained with the in situ grazing method, daily carbon rations were calculated to be 31.6% at the mouth in summer and 22.4% at Charters Creek in winter (Table 3.2).
DISCUSSION
This would limit the distribution of this key species to the Mouth/Narrows portion of the system, ie. Next important in terms of dietary contribution was POM, which contributed up to 47% of the diet at the Mond and 32% at Esengeni. An inverted salinity gradient continued, with the occurrence of hypersaline conditions in the northern regions of the estuary lake.
Beers JR (1966) Studies on chemical composition of the major zooplankton groups in the Sargasso Sea off Bermuda.
Spatial and temporal variations in the diet of the mysid Mesopodopsis africana
INTRODUCTION
The diet may include phytobenthos, phytoplankton, detritus, sediment, microzooplankton, mesozooplankton and small benthic invertebrates (Wilhelm et al. 2002, Kibirige and Perissinotto 2003a, Lehtiniemi and NordstrÖm 2008, Vilas et al.). Low freshwater input and high evaporation rates have led to a persistent reverse salinity gradient with hypersaline conditions in the upper part of the estuary. These harsh conditions, combined with the closed-mouth condition, have resulted in only the most adaptive species remaining (Carrasco et al. 2010: Chapter 1).
Lucia Estuary includes the mysid Mesopodopsis africana and the calanoid cephalopods Pseudodiaptomus stuhlmanni and Acartia natalensis (Carrasco et al. 2010: Chapter 1, Jerling et al. 2010).
MATERIALS AND METHODS
- Physico-chemical environment and microalgal biomass
- Sample collection and processing
- Stable isotope analysis
- Statistical analysis, trophic level and source contribution to diet
This sediment was then resuspended in filtered estuarine water and mixed so that the MPB remained in suspension while the heavier sediment settled to the bottom of the vessel. Samples of the dominant macroalgae (Cladophora sp.) were collected and thoroughly washed with distilled water to ensure removal of any attached sediment particles. Where the water depth exceeded 1 m (i.e. Ustje and Esengeni), the use of hyperbenthic sledges would ensure adequate collection of epibenthic mysids during the day (cf.
The mixed model SIAR v 4.0 (Stable Isotope Analysis in R) of Parnell et al. 2010) was used to determine the potential contribution of each of the potential food items to the mysid diet at both spatial and temporal scales.
RESULTS
- Autotrophic sources and physico-chemical characteristics
- Temporal variations
- Spatial variations
- Stable isotopes
- Temporal variations
- Spatial variations
- Trophic level and source contribution to diet
Despite increased rainfall and temperatures in the rainy season, there was no significant temporal difference in δ13C values of primary carbon sources in the estuary (ANOVA, F p > 0.05). In Mouth and Charters Creek, the δ13C signatures of primary carbon sources, particularly POM, were generally more enriched (+ 1 to 2‰) during the wet season (Table 4.2). In the wet season (Fig. 4.3a), the copepods Pseudodiaptomus stuhlmanni and Acartia natalensis were the most important components of the diet of mysids, contributing up to 52 and 46%, respectively.
In Mond and Esengeni, Pseudodiaptomus stuhlmanni contributed the largest proportion to the mysid diet, with 41 to 76% of the diet at Mond and 48 to 69% in Esengeni.
DISCUSSION
These low water levels and hypersaline conditions now threaten the integrity of the biotic communities present in the system. The relatively high proportion of this carbon source in the diet of mysids (especially during the dry season) may reflect the seasonal abundance of this food. Food partitioning between these species explained their ability to coexist at high biomass in the oligohalic zone of the Ebrié Lagoon (Kouassi et al. 2001).
The high proportion of Pseudodiaptomus stuhlmanni in oral Mesopodopsis africana diets may explain the apparent inverse relationship in copepod abundance relative to that of mysids.
The comparative diet of the dominant zooplankton species in the St. Lucia
INTRODUCTION
The mysid Mesopodopsis africana and the calanoid copepods Pseudodiaptomus stuhlmanni and Acartia natalensis are the dominant zooplankton taxa in St. They are important in the diet of a number of estuarine and juvenile marine fish species (Whitfield 1998). Mysid shrimp are important in the food web of aquatic ecosystems, both as consumers and producers (Mauchline 1980).
This study aims to compare the contribution of different carbon sources to the diet of the copepods Pseudodiaptomus stuhlmanni and Acartia natalensis and the mysid Mesopodopsis africana.
MATERIALS AND METHODS
- Study site
- Pelagic and benthic microalgae
- Sample collection and processing
- Stable isotope analysis
- Statistical analysis, trophic level and source contribution to diet
Models can allow the simulation of possible diets and the assessment of the relative importance of different sets of organic matter sources to consumers. Samples of the dominant macroalgae (Cladophora sp.) were collected and thoroughly rinsed with distilled water. In cases where the water depth exceeded 1 m (i.e. Ustje and Esengeni), the use of hyperbenthic sleds during the day ensured adequate collection of zooplankton showing migratory characteristics (cf.
The mixed model SIAR v 4.0 (Stable Isotope Analysis in R) from Parnell et al. 2010) was used to determine the likely contribution of each of the potential food items to the zooplankton diet.
RESULTS
- Physical characteristics
- Autotrophic sources
- Stable isotope analysis: temporal and spatial variations
- Trophic level and source contribution to diet
Regarding the contribution of the microalgal class, at Charters Creek during the wet season, 82.5% of the pelagic microalgae consisted of diatoms, dinoflagellates and chlorophytes and the remainder cryptophytes and cyanobacteria. At Munden, the microalgae were dominated by chlorophytes and cryptophytes, which together contributed up to 97% of the total microalgae present in both wet and dry seasons. At the mouth, Pseudodiaptomus stuhlmanni used mostly POM (up to 66%), with SOM, plant litter and MPB contributing up to 45, 60 and 19% of the wet season diet, respectively (Fig. 5.3a).
In the dry season, POM contributed up to 74% of the total diet with MIA, SOM and detritus contributing only up to ~45% of the diet (Fig. 5.3b).
DISCUSSION
A previous study by Carrasco and Perissinotto (2010a: Chapter 3) documented a negative relationship between copepod abundance P. These are some of the highest documented upper salinity and temperature limits for a mysid species (Murano 1966, Roast et al. et al. 2001). Furthermore, there was a negative correlation between the abundance of the copepods Peudodiaptomus stuhlmanni and Acartia natalensis and that of M.
Kibirige I, Perissinotto R (2003a) The zooplankton community of the Mpenjati estuary, a South African temporary open/closed system.
